Point-to-point radio systems that use microwaves, millimeter waves or the like are known (see, for example, Patent Literature 1). In a point-to-point radio system, two communication apparatuses perform digital communication via a point-to-point radio link. Specifically, each of the communication apparatuses is equipped with a directional antenna to communicate with a counterpart apparatus by means of a point-to-point radio technology and forms a directional beam toward the counterpart apparatus. The point-to-point radio link is thus established between the two communication apparatuses. In this specification, each of the two communication apparatuses constituting the point-to-point radio system, i.e., each communication apparatus that communicates with a counterpart apparatus by means of the point-to-point radio technology is referred to as a point-to-point radio apparatus.
In one example, a point-to-point radio system uses Frequency Division Duplexing (FDD) to perform simultaneous bidirectional communication (full-duplex communication). Further, the pair of operating radio channels that are used for the bidirectional radio links may be fixed. In this case, the point-to-point radio system uses a two radio channels with fixed frequency spacing for FDD. Each radio channel may be referred to as a radio frequency carrier.
Consider, for example, a case in which a frequency band of 59 to 63 GHz (i.e., an unlicensed band) is used for FDD and the channel spacing is set to 50 MHz. In this case, 40 channels whose central frequencies are 59.00, 59.05, . . . , 60.90, and 60.95 GHz may be used for one radio link (this link is referred to as a forward link) and 40 channels whose central frequencies are 61.00, 61.05, . . . , 62.90, and 62.95 GHz may be used for the other radio link (this link is referred to as a reverse link). Further, considering that these radio channels are used with fixed frequency spacing, as shown in FIG. 1, two radio channels with a gap of 2 GHz (e.g., a pair of 59.00 GHz and 61.00 GHz or a pair of 59.05 GHz and 61.05 GHz) can be used for the forward link and the reverse link.